1. Field of the Invention
The present invention relates to a transistor using an oxide semiconductor layer, a semiconductor device provided with the transistor, and a method for manufacturing the same.
2. Description of the Related Art
Various metal oxides exist and are used for a variety of applications. Indium oxide is a well-known material and is used as a transparent electrode material which is necessary for liquid crystal displays and the like.
Some metal oxides exhibit semiconductor characteristics. As metal oxides exhibiting semiconductor characteristics, tungsten oxide, tin oxide, indium oxide, zinc oxide, and the like are given. A thin film transistor in which such metal oxide exhibiting semiconductor characteristics is used for a channel formation region is disclosed (Patent Documents 1 to 4 and Non-Patent Document 1).
As metal oxides, multi-component oxides are known as well as single-component oxides. For example, InGaO3(ZnO)m (m: natural number) having homologous phase is a known material (Non-Patent Documents 2 to 4).
In addition, it has been confirmed that such In—Ga—Zn-based oxide can be used for a channel layer of a thin film transistor (also referred to as a TFT) (Patent Document 5, and Non-Patent Documents 5 and 6).
Further, an In—Ga—Zn-based oxide semiconductor having an amorphous structure is formed more easily than zinc oxide (ZnO) having an amorphous structure. Thus, an In—Ga—Zn-based oxide semiconductor is used for a channel layer of a transistor, whereby characteristic variation among transistors can be reduced even when a substrate to be manufactured has a large area and the transistors can be normally off. Meanwhile, in a case where an oxide semiconductor layer including an amorphous structure is used for a channel layer, there is a problem in that contact resistance between the oxide semiconductor layer and a source and a drain electrode layer is increased.